The demon king is a high schooler(High School Dxd x 7ds x male autistic reader)

In the Arthurian legends king Arthur used Excalibur to become king. that's only a half of the story. he obtain excalibur from a god and used it to fight the most powerful demon who ruled the other demons even the gremory familly feared who was also the son of this same god. after the fight Arthur saw how much dangerous the demon was and with the help he sealed the soul of the demon in excalibur before seperating excalibur into 7 swords that he sent across the world in hope hat he will never be back. But unknown to Arthur a part of the soul of the demon was kept by the god. He then waited that the time wiped the memory of this battle and then reincarnated the soul of the demon. I wounder how his life will go